Reusing dirt:

I personally reuse my dirt as much as possible partly due to my failure rate (with seedlings) partly cause I'm cheap and I figure "why not?" 

I just found some melon growing in with the peppers I started as seedlings and I know it's from my collection somewhere I moved it to another area where I can "watch" it a bit more closely but I think in the process I killed the root so hopefully there's enough and it lives 

Another example is this guy I know the big ones sunberry but I have no clue about the small ones they look similar and I don't know deadly nightshade to come up that much at once in an area (wild around here) so I'm hoping it's the sunberries I threw in the dirt last year 

I've also added some Epsom salts and couple ferts to everyone (well epsoms went to tomatoes and peppers and one pepper looks growing like mad! Like a million flowers!) that's why that tomato is more green from nowhere